This proposal seeks to further understanding of an essential component of the skin immune system, the cathelicidin antimicrobial peptides. The absence of cathelicidin is associated with disease in animals, and infections in humans with atopic dermatitis. The precise identity of the cathelicidin peptides in normal and diseased.skin is not known. Also unknown are the host proteases produced in the skin that are responsible for the activation of cathelicidins and the bacterial proteases that degrade cathelicidins. to enhance their virulence. The current proposal will test the hypothesis that the skin generates a unique array of cathelicidin peptides by a strictly regulated system of proteolytic activation that is in balance with pathogen proteases. Understanding proteolysis of cathelicidins will provide a frameworkfor modification of function to enhance resistance to infection and modify potential deleterious host inflammatory effects of these peptides. The Specific Aims are: 1. Identify cathelicidins peptide products produced by keratinocytes and whole skin a. Identify peptides generated by cultured keratinocytes b. Identify and quantify peptides in normal skin and psoriasis 2. Define cathelicidin processing in skin by identifying keratinocyte-specific processing enzymes. a. Purify enzymes responsible for cleavage of hCAP18 to release the cathelin prodomain b. Purify enzymes that cleave cathelicidin peptides to products determined in Aim 1 c. Confirm enzyme activity in cell and animal models 3. Determine if cathelicidin proteolysis is a virulence mechanism for an invasive bacterial skin pathogen a. Fully characterize GAS proteolytic degradation of cathelicidins and corresponding AMP resistance b. Determine the contribution of cathelicidin degradation to GAS virulence in invasive skin infection c. Assess if the cathelin prodomain serves to protect mature cathelicidin from proteolytic degradation Accomplishing the aims of this proposal will impact public health by providing a greater understanding of how we fight microbial infections and how bacteria try to avoid our immune system. This work will suggest novel approaches for fighting infectious disease of the skin.